Gnosticism, which gets its name from the Greek word <gnosis>
("knowledge") was a religious movement beginning, possibly, before the
time of Christ and extending into the first few centuries of the Christian era.
Gnostics viewed themselves as "those who know." Their heretical
teachings varied from group to group and can't be pinned down with specificity,
but common gnostic beliefs included these:

Although Christ appeared to be human, his humanity was merely an illusion.

Christ appeared to die, but did not really die. The Crucifixion was really a
cruci<fiction>.

Christ was not truly God, the second Person of the Trinity. He was merely a
created being who was the lowest of the <aeons>, a group of semi-divine
beings between God and man. Each lower <aeon> was given power by a higher
<aeon>. Christ, the <aeon> furthest removed from God, created the
world because God was too pure to dirty himself with matter.

Matter is evil, so one can do anything one wants with one's body, including
killing it to release the soul from its imprisonment.

The God of the Old Testament is evil, as evidenced by the fact that he
created the material universe. He is not the same as the God of the New
Testament, who is the God of Love, as Jesus and his apostles taught (1 John 4:8,
16).

People are saved by acquiring secret knowledge (<gnosis>), which is
imparted only to the initiated. Gnosticism was similar in some ways to the
modern New Age movement. Like New Agers, gnostics used Christian terminology and
symbols, but placed them in an alien religious context that gutted the essential
teachings of Christ. It's unclear when gnosticism began. Many Church Fathers
thought gnosticism was founded my Simon Magus, the Samaritan sorcerer who
converted to Christianity (Acts 8:9-24). Some contemporary scholars think
gnosticism started a few centuries before Christianity and then invaded it from
the outside through the conversion to Christianity of Jewish and Gentile
gnostics. Other scholars believe gnosticism started as a Christian heresy.

It seems clear, though, that the apostles themselves had to contend with a
form of gnosticism (Col. 2:8, 18, 1 John 4:1-3, Rev. 2:6, 15). Paul said,
"Avoid profane babbling and the absurdities of so-called knowledge
[<gnosis>]. By professing it some people have deviated from the
faith" (1 Tim. 6:20-21).